Part: move icons to booleans subdirectory

The path to the icons is added to `AppPartGui.cpp`
using `Gui::BitmapFactory().addPath()`, so the icons are found
automatically in their new directories.

src/Mod/Part/Gui/AppPartGui.cpp

@@ -125,6 +125,7 @@ PyMOD_INIT_FUNC(PartGui)
 
     Base::Console().Log("Loading GUI of Part module... done\n");
 
+    Gui::BitmapFactory().addPath(QString::fromLatin1(":/icons/booleans"));
     Gui::BitmapFactory().addPath(QString::fromLatin1(":/icons/create"));
     Gui::BitmapFactory().addPath(QString::fromLatin1(":/icons/tools"));